The premise it's all wrong. The reason we let an "innocent" person go free it's because you don't have enough evidence to prove that it is guilty in the first p

The premise it's all wrong. The reason we let an "innocent" person go free it's because you don't have enough evidence to prove that it is guilty in the first place. Imprisoning that person anyway just to be "safe" won't make you safer since the actual guilty person may still be around. If that is the case then you effectively pardoned him/her while exacting punishment on an innocent. It's very easy to talk about the "innocent" person in the abstract but if you happen to be the innocent you will care very much about being punished without having actually committed any crimes.